<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>

    <style>
       /* .parent{
            text-align: center;
            width: 500px;
            height: 500px;
            background: red;
        }
        .child{
            display: inline-block;
            width: 300px;
            height: 300px;
            background: green;
        }*/水平居中

    </style>
</head>
<body>
<div class="parent"><div class="child"></div></div>
</body>の41
</html>